The present invention relates to an optical probe to produce or to detect near-field light for use with a near-field optical microscope or a near-field optical recording/reproducing device.
An optical microscope employs a lens to collect or to condense light. In this system, resolution is restricted by a wavelength of the light. In contrast with this optical microscope, a near-field optical microscope uses a near-field optical probe which produces optical near-field location in the vicinity of the probe in place of the lens. The near-field optical probe is placed in the neighborhood of a sample to scan a surface of the sample. It is resultantly possible to measure a contour and optical characteristics of the sample with resolution determined by the size or dimension of the aperture. The near-field optical microscope has been recently applied to various fields such as a field of contour measurement and spectroscopic analysis of, for example, a sample of an organism, quantum structure of a semiconductor, and a macromolecular material as well as a field of high-density optical recording.
As the near-field optical probe, a pointed optical fiber (optical fiber probe) having a fine opening of a size less than a wavelength of light has been usually employed. To fabricate this fiber probe, a tip end section of an optical fiber is extended while being heated. Alternatively, the tip end section is tapered to a point by chemical etching. Thereafter, the optical fiber other than the tip end section is coated with metal. By introducing light to the optical fiber, near-field light can be generated in the proximity of an aperture formed in the tip end section.
However, this fiber probe is attended with a drawback of low light utilization efficiency. When light is incident to a fiber with a fiber probe of this kind having, for example, a diameter of 100 nm, intensity of light emitted from the tip end of the fiber is about 0.001% or less of that of light incident to the fiber. To overcome this problem, various probes have been proposed as follows. (1) Multi-step tapered fiber probe: A fiber probe having a tip end section which is tapered in two or three steps to a point (Applied Physics Letters, Vol. 68, No. 19, pp. 2612-2614, 1996 and Vol. 73, No. 15, pp. 2090-2092, 1998), (2) Metallic needle probe: A probe of a needle of STM. By emitting light to a tip end section of the needle, strong near-field light is produced in the vicinity of the tip end (JP-A-6-137847). (3) Fiber probe with small metallic particle in aperture: A fiber probe in which a very small metallic particle is disposed at a center of an aperture in a tip end section (JP-A-11-101809 proposed by the first inventor of the present invention). Light emitted from the aperture excites plasmon in the small metallic particle to produce strong near-field in the neighborhood of the small metallic particle. (4) Tetrahedral tip: A triangular prism of glass is coated with metal having a thickness of about 50 nm so that surface plasmon is excited on the metal film. The surface plasmon proceeds toward a top end or a vertex of the triangular prism to produce strong near-field light in the proximity of the vertex (Physical Review B, Vol. 55, No. 12, pp. 7977-7984, 1997). (5) Probe on glass substrate with metallic scatterer: A probe including a glass substrate and a metallic scatterer formed on a bottom surface of the glass substrate. This configuration generates strong near-field light in the proximity of the metallic scatterer (JP-A-11-250460).
In the near-field optical microscope, it is necessary to set distance between the aperture to generate near-field light and a surface of a sample to a value ranging from several nanometers to several tens of nanometers. consequently, when the probe including an optical fiber or a glass piece is used, a particular control system is required to control the distance between the tip end of the probe and the sample surface. In general, the distance is measured using interatomic force between the tip end of the probe and the sample surface, and the distance is adjusted by servo control using the measured value.
However, the servo control has a limited servo band and hence the probe scanning speed is limited. Particularly, in an optical recording/reading device to operate at a high data transfer speed, the probe must scan a surface of a recording disk at a high speed. This method cannot appropriately control deviation of an interval of a high frequency caused by distortion and inclination of the disk. To solve this problem, various probes have been proposed as follows. (1) Flat opening probe: A probe fabricated by disposing an opening in a silicon substrate by anisotropic etching (The Pacific Conference on Lasers and Electro-Optics, WL2, xe2x80x9cFabrication of Si planar apertured away for high speed near-field optical storage and readoutxe2x80x9d. Since a peripheral area of the aperture is flat, the distance between the probe and the sample can be kept fixed by pushing the probe against the sample. (2) Probe with aperture having pad: On a bottom surface of a glass substrate, a projection in the form of a quadrangular prism having an aperture in a tip end thereof is fabricated, and a pad is manufactured in a periphery of the projection (JP-A-11-265520). The pad keeps the distance between the probe tip end and a sample. (3) Surface emitting laser probe with small metallic tip: On a laser emitting end surface of a surface emitting laser probe, a small opening and a small metallic projection are fabricated (Applied Physics, Vol. 68, No. 12, pp. 1380-1383, 1999). Since the probe has a flat structure, the distance between the probe and a sample can be kept fixed by pressing the probe against the sample. The probe has a small metallic projection and a resonance structure, the probe expectedly operates with higher efficiency.
The near-field probe requires three points regarding performance as follows. (1) High light utilization efficiency, (2) High scanning, and (3) Reduced background light in light measured by the probe.
To increase the light utilization efficiency, various methods have been proposed as above. The fiber probe having a tip end with multiple taper angles has light utilization efficiency which is about ten times to about one hundred times that of a fiber probe generally used. However, this probe is not fully applicable to applications requiring high light utilization efficiency, for example, to the optical recording/reading requiring a light utilization efficiency of 10% or more. The probe uses an optical fiber and is mechanically fragile and cannot scan at a high speed. The metallic needle probe, the fiber probe with small metallic particle in aperture, the glass probe coated with metal, and the probe on glass substrate with metallic scatterer have increased light utilization efficiency by using characteristics of metal, and hence a high light utilization efficiency can be expected. However, each of these probes has a tip end section with a mechanically fragile contour and hence is not suitable for the high-speed scanning. Particularly, in operation of the metallic needle probe and the probe on glass substrate with metallic scatterer, light which does not hit the tip end section or the scatterer is also incident to the sample. This resultantly leads to a problem of detection of much background light.
Various probes capable of scanning at a high speed have been proposed as above. The flat opening probe and the probe with aperture having pad can achieve the high-speed scanning. However, these probes have low light utilization efficiency. The surface emitting laser probe with small metallic tip expectedly scans at a high speed with high light utilization efficiency and a low amount of background light. To generate strong near-field light using the small metallic projection, the contour of the small metallic projection must be optimized. However, description has not been given of the contour of the small metallic projection at all. Moreover, description has not been given of a method of manufacturing the small metallic projection.
It is therefore an object of the present invention to provide a near-field optical probe and a method of manufacturing the same which satisfies three requirements above, that is, high light utilization efficiency, high-speed scanning, and little background light in light measured by the probe. Another object of the present invention is to improve the light utilization efficiency by particularly using a metallic scatterer having a size equal to or less than the light wavelength and therefore to provide an optical contour of the scatterer and a method of supplying light to the probe to improve the light utilization efficiency.
According to the present invention, there is provided a near-field optical probe including a substrate, a metallic scatterer fabricated on the substrate in a contour of a circular cone, a triangle, or the like; and a film of a metal, a dielectric, or a semiconductor manufactured in a periphery of the scatterer, the film and the scatterer being substantially equal in height to each other. The metallic scatterer is used to generate strong near-field light, and the film in the periphery thereof is disposed to prevent destruction of the scatterer when the probe is placed in the proximity of a sample to scan the sample at a high speed. By using a light shielding material for the film and by setting the distance between the scatterer and the film to a value equal to or less than the light wavelength, the film functions to reduce background light. To prevent the destruction of the scatterer, there may be manufactured, in place of the fabrication of the film, a dip or recess in a surface of the substrate with depth substantially equal to height of the metallic scatterer. The metallic scatterer is formed in the dip. To further reduce probability of destruction of the scatterer, a light transmitting film may be filled in a gap between the scatterer and the peripheral film and between the scatterer and the dip fabricated on the substrate surface.
The metallic scatterer has a contour of a circular cone, a polygonal pyramid, an ellipse, or a triangle. When the scatterer has a triangular contour, each of two vertices of the triangle may have a radius of curvature larger than that of the other one vertex of the triangle. The film of the triangle may be connected to that of the periphery. In this case, the hole or opening has, at the connecting area, a radius of curvature larger than those of the vertices of the triangle. In fabrication of the scatterer, a metallic film may be formed on a substrate with a tip end in a contour tapered to a point such as a planar ellipse or a planar triangle. In the vicinity of the tapered tip end, another metallic film is manufactured such that the distance between the tip end and the metallic film is equal to or less than the light wavelength. Particularly, it is favorable to fabricate two metallic films each having a tip end section with a tapered contour such that the distance between two tip ends of the metallic films is equal to or less than several tens of nanometers. When the scatterer is formed with a metallic film having a tip end section with a contour of a triangle, an ellipse, or the like; the metallic film may be manufactured on a side surface of the substrate.
The planar or flat substrate may be replaced with a hemispherical substrate to minimize a spot diameter at a focal point of incident light. On the substrate, there may be arranged a light condensing device such as a holographic lens. A metallic scatterer may be disposed on a light emitting edge surface of an optical resonator or a semiconductor laser. When the scatterer includes a film in the contour of an ellipse or a triangle, the film of the ellipse or the triangle may be fabricated on a side surface of the substrate or an inclined surface of the substrate so that only one intersection between a major axis of the ellipse and the ellipse or only one vertex of the triangle is brought into contact with a surface of a sample.
According to the present invention, there is provided a method of manufacturing a near-field optical probe comprising a film forming step of forming a film of a metal, a dielectric, a semiconductor on a substrate, a resist coating step of fabricating a resist film on a film, an exposure and development step of removing the resist film of an area in which a scatterer is to be fabricated, a film etching step of removing part of a film, a scatterer forming step of manufacturing a metallic scatterer in the area from which the resist has been removed, and a resist removal step of removing the resist film. In production of a scatterer in a contour of a circular cone or a polygonal pyramid, the area to remove the resist has a contour of a circle with a diameter equal to or less than a wavelength of light or a contour of a polygonal pyramid with an edge having a length equal to or less than a wavelength of light. In the scatterer forming process, metal is evaporated thereon, and the metal is thick enough to completely cover the hole in the circular or polygonal shape. In the manufacturing the near-field optical probe, the film forming step may be removed such that a substrate etching step to etch the substrate is used in place of the film etching step.
To manufacture a near-field optical probe according to the present invention, there may be employed a manufacturing method including a film forming step of forming a film of a metal, a dielectric, a semiconductor on a substrate, a dip forming step of removing part of the film by photolithography or the like, a resist coating step of fabricating a resist film, an exposure and development step of removing the resist film of an area in which a scatterer is fabricated, a scatterer forming step of manufacturing a metallic scatterer in the area from which the resist has been removed, and a resist removal step of removing the resist film.
In the manufacturing of the probe above, in place of the dip forming step of removing part of the film by photolithography or the like, there by be employed a dip forming step of directly forming a dip in a surface of the substrate by photolithography or the like.
Moreover, the near-field optical probe may be produced in a manufacturing method including a metallic film forming step of forming a film of a metal on a substrate, a resist coating step of fabricating a resist film on the metallic film, an exposure and development step of removing the resist film of a peripheral area in which a scatterer is fabricated, a metallic film etching step of removing the metallic film from the area from which the resist has been removed, and a resist removal step of removing the resist film.
The near-field optical probe may be produced in a near-field optical probe manufacturing method including a resist coating step of fabricating a resist film on a substrate, an exposure and development step of removing the resist film of a peripheral area in which a scatterer is to be fabricated, a metal evaporation step for fabricating a scatterer, and a resist removal step of removing the resist film.
According to the present invention, a near-field optical probe in which the scatterer is protected by an dielectric substance is produced by a manufacturing method including a dielectric film forming step of fabricating a scatterer and a film in a periphery of the scatterer and then manufacturing a dielectric film thereon and a dielectric film polishing step of polishing the dielectric film such that a tip end section of the scatterer exists in a surface region.
To introduce light to the near-field optical probe of the present invention, the focal point of incident light must be at a position of the scatterer. For this purpose, there is employed an automatic focal point adjusting method in which part of light incident to the near-field optical probe is separated, the separated light is fed to a focal point adjusting pattern disposed next to a source of the near-field light to measure a contour of light reflected on the adjusting pattern, and the focal position is adjusted according to a result of the measurement. Particularly, in the focal point adjusting in a direction vertical to the substrate surface, a beam of light reflected from the focal point adjusting pattern is delivered to a convex lens and a cylindrical lens to measure distortion of a contour of a light beam delivered thereto. In the focal point adjusting in a direction parallel to the substrate surface, there is fabricated a focal point adjusting pattern including two small elongated grooves which each have width less than a diameter of a pertinent light spot and which vertically intersect each other. The incident light is divided into three beams of light. A first beam enters the source of near-field light and second and third beams enter a central area of two grooves. Patterns of reflection light from two grooves are measured to detect two bright areas respectively of the patterns. Quantity of light is compared between the bright areas.
When the near-field optical probe of the present invention is applied to an optical recording/reading device in which disks can be changed, it is required to prevent dirt and damage on a surface of the disk. In the optical recording/reading device according to the present invention, a near-field optical probe is incorporated in a cartridge which protects the recording disk. The cartridge has a rotary shaft at a corner thereof. An arm is attached to the rotary shaft. The near-field optical probe is installed on the arm using a suspension. Coupled to the rotary shaft with the arm is an arm on which an optical head including a light source and a light sensor or detector is attached. The optical head moves in cooperation with the near-field optical probe. Light from the optical head is incident to the near-field optical probe via a window disposed in the cartridge. To couple the arm linked with the near-field optical probe with the arm linked with the optical head, a v-shaped groove and a semi-spherical projection are used. When the scatterer includes a metallic projection in the form of a circular cone or a polygonal pyramid or a metallic film fabricated on a side surface of the substrate with a tip end section tapered to a point in the form of a triangle or an ellipse, a metallic film is favorably fabricated below a recording layer of the recording disk to improve resolution and efficiency.